Opportune Shot (Ex)Sharpshooters always take a keen eye on the battlefield, watching for opportunities to fire at distracted targets. They can make ranged attack of opportunities at targets within their first range increment whenever the target makes an attack that's not aimed at the Sharpshooter. Once a Sharpshooter has used this ability against a target, he can't target the same target again with it until the start of his next turn. This ability can't grant the Sharpshooter more attacks per round than his Sharpshooter level. (Start counting at the start of the Sharpshooter's turn, only attacks made using Opportune Shot count for this limit.) Additionally, the Sharpshooter gets 1d4 bonus precision damage on all ranged attacks of opportunity. This bonus damage increases by 1d4 at 3rd level and every two levels thereafter. When attacking creatures immune to critical hits, the sharpshooter instead deals additional damage equal to their dexterity bonus or character level, whichever is worse. Starting at 8th level, the sharpshooter additionally adds his Opportune Shot damage to ranged attacks that come from readied actions.

Precise Shot: Bonus Feat

Combat Reflexes: Bonus Feat

Defensive Shot (Ex)Starting at 2nd level, the Sharpshooter no longer threatens when making ranged attack rolls against enemies within the first range increment. At 6th level, and every 4 levels thereafter, this effect increases by one range increment. (Two range increments at 6th, three at 12th, etc.)

GambitsAt every even level, the Sharpshooter learns a new Gambit. The Sharpshooter must meet the requirements listed in the entry. Unless otherwise noted, these abilities are Extraordinary and require a standard action to use. See the Gambits section for more detail.

Shoot the Weak Spot (Ex)At third level, the Sharpshooter gains the ability to hit weak points even in those enemies with damage reduction. They subtract their dexterity bonus from their enemy's DR with all ranged attacks. Creatures immune to critical hits and precision damage don't have their DR lowered by this class feature.

Point Blank Shot: Bonus Feat

Read the Wind (Ex)Starting at fifth level, a Sharpshooter can make efficient use of the flow of harsh winds when making ranged attacks. His ranged attacks are treated as Siege Weaponry for the purpose of Wind and Wind-based effects if it's beneficial for him. Starting at 15th level, he can choose to ignore Wind and Wind-based effects entirely when making ranged attacks.

Far Shot: Bonus Feat

Critical Shot (Ex)Starting at 11th level, the Sharpshooter is considered to possess Improved Critical for all ranged weapons he's proficient with, but only in circumstances where he'd get to add his Opportune Shot Damage. Furthermore, he adds his dexterity bonus multiplied by the weapon's critical modifier to damage on critical hits in the same circumstances, even if the target is immune to critical hits. (Although other benefits for critical hits don't apply to targets immune to critical hits.)

Piercing Shot (Ex)Starting at 13th level, whenever there's multiple targets in any single line that has the Sharpshooter at one end, and there's no other obstacles between the Sharpshooter and the targets, he can try to hit all of them. The closest target is treated as being at its normal range increment, all other targets are considered a number of range increments further away than they really are equal to the amount of targets that were closer in the line to the sharpshooter.

Gambits

Arrow ClashRequirement: BAB +10Whenever someone makes a ranged attack on you, or a ranged attack's trajectory passed by within 30 feet of you, you can choose to intercept the attack as an attack of opportunity. Make an attack roll against the attack. The Attack's AC is 20 + the attacker's dexterity bonus. If you hit the attack, the attack stops there. If the attack is a grenade-like weapon its splash effects consume the arrow, but otherwise have no effect.

Caster's BaneIf someone casts a spell or uses a spell-like ability, within 30 feet of you, they provoke a ranged attack of opportunity unless they cast defensively.

Evasive MovementAs a Move action, you can move up to half your speed without provoking attacks of opportunity for moving.

Suppressive FireAs a standard action, Choose a line with a length equal to the range increment of your ranged weapon. One end of the line must be your square. You cannot move until the start of your next turn. Until the start of your next turn, whenever someone moves into or out of that line, you get a ranged attack of opportunity against that person.

It looks fairly good for Tier 3-ish balance, playing with warblades and such. There should probably be wording that qualifies the sharpshooter for Stand Still and similar AoO-based feats.

For gambits, you might consider an ability that temporarily takes away the ability to fly (pinning back a creature's wings or something) and, at higher level, an upgrade to Caster's Bane that lets you dispel a single effect (appropriately limited to prevent IHS shenanigans) with a ranged attack.